Codec
The compression scheme used to encode and decode video data.
In depth
Codecs trade file size against quality and processing load. Delivery codecs achieve small files with heavy compression that is difficult to edit; intermediate codecs use lighter compression and edit smoothly at much larger sizes. Choosing the wrong one for the task is the commonest cause of an unworkable edit.
Examples in practice
Transcoding heavily compressed camera files to an intermediate codec turns stuttering playback into smooth editing.